When you import, you have the option (I just discovered 5 minutes ago) to have Lightroom render 1:1 previews. That makes it much easier to just quickly flip through a bunch of images, because it doesn't have to try to render them on the fly behind your back.

However, I have a bunch of images that are already in Lightroom which are not rendered at 1:1 scale. I'd like to have Lightroom render 1:1 previews for itself of those.